<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
	</head>
	<style type="text/css">
		#app{
			
			
		}
		#app span{
			
		
		}
	</style>
	<body>
		<div id="app">
			<span>垂直居中</span>
		</div>
	</body>
</html>
